In the typical powder metal compacting press, the die and lower tooling form a cavity into which the powder metal is deposited by an appropriate feed mechanism.
As is well known, a number of variables can affect the part weight for a given volume cavity. Such variables include feeder time over the die, mechanical action to the powder, geometry of the cavity, flowability of the powder and apparent density of the powder. Of the above variables, the first three are currently being controlled by good machine design and good tooling and part design. These three factors tend to affect the part-to-part weight variation but remain constant throughout a production run. Some part-to-part variation is unavoidable.
The present invention is primarily concerned with a trend variation. A trend variation, as opposed to a part-to-part variation, may be defined as a continuing shift of the average of the last ten tonnages away from the target tonnage. The selection of 10 tonnages is somewhat arbitrary and non-limiting, but has been found to be sufficient to determine the start of a trend variation. It is important to note that the trend variation may be determined in terms of a continuing shift of the average of the last 10 press tonnages away from the target press tonnage or as a continuing shift of the average of the part weight of the last 10 parts away from the target part weight. It has been found that there is a very strong linear relationship between press tonnage and part weight so that either can be used in the determination of a trend, as will be apparent hereinafter.
Of the above-named variables, flowability of the powder and apparent density of the powder are primary contributors to trend variation and are the most difficult to monitor and control before a part is made. It has long been known that powder metal flowing by gravity into a cavity will not always settle into the exact same apparent density. There are other factors which contribute to trend variation. These include die temperature change, the ambient temperature and the humidity, inconsistent head of powder in the final hopper systems, and source air starvation that changes platen motion.
Heretofore and up to the present time, reliance was placed on the operator to watch over the output of the press and to make as many adjustments as necessary to keep the part within specification limits. Added to the difficulty of this human monitoring, is the fact that when fill adjustments are called for, they are frequently incorrectly made for a multi-level part.
Proper fill adjustment is extremely important. The tool designer designs each tooling level to deflect to the same amount when the loads are balanced. Balanced loads and tool deflections are essential to a part that is to be free of powder shear cracks or ejection cracks and that is to have the proper densities. The set-up person spends a great deal of time setting up a multi-level job so as to arrange the fill on each level such that the loads on each punch are at specific tons per sqaure inch. A tooling level that sees a lower load than designed will leave its section of the part unsupported during ejection, which may cause the part to crack. The tool designer also designs the tooling so that various portions of the part will be characterized by the proper densities. Similarly, a tooling level that has a higher than designed load applied to it will leave the adjacent levels of the part unsupported during ejection, again possibly causing the part to crack. This critical balance and the proper densities are frequently lost during a production run because of improper fill adjustments causing the load balance to change between levels. Frequently, improper fill adjustments are made by varying the fill position of only one level. A fill adjustment for one level is fine for a single level part, but will often lead to difficulties with respect to a multi-level part. It is obviously not practical to expect an operator to calculate the proper fill adjustments necessary for each level each time a fill adjustment is required.
It is a primary object of the present invention to provide an automated system for determining when powder fill adjustments are required and to properly make these adjustments. To put in an automated system to monitor and control all of the variables which may contribute to trend variations would be extremely complex and cost prohibitive. The automated system of the present invention monitors the result and effect of these variables as reflected by weight changes in the compacted parts or compacting load changes which reflect such weight changes. The phrase "proportionately correct adjustments" is intended to mean adjusting the fill positions to retain the original relationship between each column of powder such that the percent change in powder column height is the same for all columns.
In recent times, part specifications continue to get tighter and tighter. At the same time, more presses are running unattended than ever before. As a result, it is believed that the system of the present invention fills a significant need in the industry.